聲射流(acoustic streaming)是一種經由高頻振盪，於流場中固液介面處進行交互作用產生的穩態流動現象。本研究旨在探討三角形微結構所誘發之聲射流現象，聲射流之產生是透過壓電傳感器作為振動源將振動傳入流體，並以流場可視化(Flow Visualization)及微粒循跡測速儀(Particle Tracking Velocimetry, PTV)技術對微結構引致之穩態渦旋進行探討。本研究探討兩種微流體幾何設置，一種是以微影製程(photolithography)方式製作母模，並在流道側壁上設計五種不同頂角(α=28°,41°,53°,90°,127°)之對稱三角形結構和三種不同頂角(α=20°,35°,70°)及傾角(β=30°,45°,60°)之非對稱三角形結構，再以翻模成型(Micro-molding)翻製PDMS流道。實驗中振盪頻率為1kHz ~ 12kHz，並利用12V、20V及30V驅動電壓，來觀察聲射流的流動型態及其速度場與渦度場的變化。實驗結果顯示，於結構尖端左右兩側會產生一對流動方向相反之聲射流渦旋，而其中不論是對稱三角形或是非對稱三角形，其頂角(α)角度與最大絕對速度值及渦旋範圍之關係大致為負相關，角度越小時，產生的聲射流渦旋較穩定，於流場中影響的範圍也較廣，且最大絕對速度值也較快。此外，聲射流渦旋的流動模式會隨著非對稱三角形之傾角變化而有所傾斜。而當電壓提升時，聲射流流場所產生的渦旋流動越快，渦旋範圍也越大。不同的振盪頻率與聲射流渦旋範圍，兩者間並不是呈線性關係，於低頻所誘發之聲射流渦旋呈左右對稱的形式流動，而在高頻所誘發之聲射流渦旋形式較不對稱。從不同流場觀測高度下之可視化結果，可推測在靠近PDMS這一側流動仍維持二維流場。整體而言，針對三角形微結構所誘發之聲射流渦旋，當改變結構之頂角及驅動電壓對於流場的影響是較為顯著的。第二種微流體裝置則是藉由在流道內放置含有三角形結構之黃銅金屬薄片並使結構尖端保持懸空，以減少三角形微結構之聲阻抗傳遞。透過高頻振盪此裝置時，於不同高度下進行流場可視化觀察，並探討三角形結構之頂角角度變化(α=28°,41°,53°,90°,127°)對於三維流場的影響。實驗結果顯示，沿著薄板左右兩側之邊緣會產生一系列的渦旋，而其旋轉軸平行於三角形之邊緣並於頂角處形成複雜之三維流動。

Acoustic streaming is a steady flow phenomenon induced by high frequency oscillation of the solid-liquid interface in the flow field. The purpose of this study is to investigate the acoustic streaming phenomenon induced by the triangular microstructures, in which the oscillation is transmitted to the fluid through the piezoelectric transducer. The steady streaming vortices generated around the microstructure are investigated by flow visualization (FV) and particle tracking velocimetry (PTV) technique. Two microfluidics configurations were investigated in this study. The master mold of the first configuration is made by the photolithography method with different designs of triangular microstructure on the sidewall of the flow channel, including symmetrical triangular structures of five different tip angles (α=28°, 41°, 53°, 90°, 127°), asymmetrical triangular structure of three different tip angles (α=20°,35°,70°) and inclined angles (β=30°, 45°, 60°). The PDMS microchannels are then made by micro-molding from the master mold. In the experiments, the range of oscillation frequency is 1kHz ~ 12kHz, and 12V, 20V and 30V driving voltages were used to observe the flow pattern of the acoustic streaming and the changes of its velocity field and vorticity field. The experimental results show that a pair of acoustic streaming vortices with opposite flow directions are generated on the two sides of the tip of the structure. Whether symmetrical triangle or asymmetrical triangle, the tip angle is inversely correlated to its maximum absolute velocity value and the vortex range. When the angle is smaller, the acoustic streaming vortex generated is more stable, the region of influence in the flow field is wider, and the maximum absolute velocity value is also larger. In addition, the flow pattern of the acoustic streaming vortex changes with the inclined angle of the asymmetric triangular microstructure. When the voltage increases, the vortical flow generated by the acoustic streaming is faster, and the region of the vortical flow is larger. There is no linear relationship between the different oscillation frequency and the region of the acoustic streaming vortex. The acoustic streaming vortices induced at low frequency flow in a symmetrical form, while those induced at high frequency are more asymmetrical. From the visualization results under different observation heights of the flow field, it can be inferred that the acoustic streaming induced by this configuration maintains 2-D flow near the PDMS side of the channel. The second configuration of the microfluidic device is made by placing a triangular-shaped brass metal sheet into the flow channel and keep the tip of the structure suspended to reduce the acoustic impedance of the triangular microstructure. Flow visualization at different heights of the channel in the device were observed when the device was oscillated at high frequency to investigate the change of tip angles (α=28°,41°,53°,90°,127°) of the triangular microstructure and the resulting 3-D flow fields. The experimental results show that a series of vortices are generated along both edges of the triangular microstructure, and their rotational axes are parallel to the edges. Complicated 3-D flow patterns are also formed at the tip of the triangular microstructure.

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